The present invention relates to an active matrix display device in which driving of luminescent elements such as LEDs (light emitting diodes) or EL (electroluminescent) elements which emit light when a driving current is passed through an organic semiconductor film is controlled by thin film transistors (hereinafter referred to as TFTs). More specifically, the present invention relates to a technique of optimizing the layout to improve the display performance.
Active matrix display devices using current-controlled luminescent elements such as EL elements or LEDs have been proposed. Any types of luminescent elements used in display devices of this type have the capability of emitting light. Therefore, backlight is not required in display devices of this type unlike the liquid crystal display devices.
FIG. 13 is a block diagram illustrating an example of such an active matrix display device in which carrier injection type organic thin film EL elements are employed. The display device IA shown in this figure includes various elements formed on a transparent substrate, such as a plurality of scanning lines xe2x80x9cgatexe2x80x9d, a plurality of data lines xe2x80x9csigxe2x80x9d extending in a direction crossing the direction in which the plurality of scanning lines xe2x80x9cgatexe2x80x9d extend, a plurality of common power supply lines xe2x80x9ccomxe2x80x9d extending in a direction parallel to the data lines xe2x80x9csigxe2x80x9d, and pixel regions located at respective intersections of the data lines xe2x80x9csigxe2x80x9d and the scanning lines xe2x80x9cgatexe2x80x9d. To drive the data lines xe2x80x9csigxe2x80x9d, there is provided a data line driving circuit 3 including a shift register, level shifters, video lines, and analog switches. Similarly, to drive the scanning lines, there is provided a scanning line driving circuit 4 including a shift register and level shifters. Each pixel region 7 includes a first TFT 20 having a gate electrode to which a scanning signal is supplied via a scanning line, a holding capacitor xe2x80x9ccapxe2x80x9d for holding an image signal supplied from a data line xe2x80x9csigxe2x80x9d through the first TFT 20, a second TFT 30 having a gate electrode to which the image signal held by the holding capacitor xe2x80x9ccapxe2x80x9d is supplied, and a luminescent element 40 into which a driving current flows when the luminescent element 40 is electrically connected to a common power supply line xe2x80x9ccomxe2x80x9d via the second TFT 30.
In each pixel region, as shown in FIGS. 14(A) and 14(B), the first TFT 20 and the second TFT 30 are formed using two respective island-shaped semiconductor films wherein one of the source/drain regions of the second TFT 30 is electrically connected to an interconnecting electrode 35 via a contact hole formed in a first interlayer insulating film 51 and the interconnecting electrode 35 is electrically connected to a pixel electrode 41. At upper layers above the pixel electrodes 41, there are provided a hole injection layer 42, an organic semiconductor film 43, and an opposite electrode xe2x80x9copxe2x80x9d, which are formed in a multilayer structure. The opposite electrode xe2x80x9copxe2x80x9d extends across the data lines xe2x80x9csigxe2x80x9d and other lines over a plurality of pixel regions 7.
The other one of the source/drain regions of the second TFT 30 is electrically connected to the common power supply line xe2x80x9ccomxe2x80x9d via a contact hole. On the other hand, in the first TFT 20, one of the source/drain regions is electrically connected to a potential sustaining electrode xe2x80x9cstxe2x80x9d which in turn is electrically connected to an extension 310 of the gate electrode 31. A semiconductor film 400, which is doped with an impurity so that it exhibits conductivity, is disposed below the extension 310 such that the semiconductor film 400 and the extension 310 face each other via a gate insulating film 50. As a result, a holding capacitor xe2x80x9ccapxe2x80x9d is formed with the extension 310, the gate insulating film 50, and the semiconductor film 400. The semiconductor film 400 is electrically connected to the common power supply line xe2x80x9ccomxe2x80x9d via a contact hole formed in the first interlayer insulating film 51. The holding capacitor xe2x80x9ccapxe2x80x9d holds the image signal supplied from the data line xe2x80x9csigxe2x80x9d via the first TFT 20 so that the gate electrode 31 of the second TFT 30 is maintained at a potential corresponding to the image signal even after the first TFT 20 is turned off. As a result, the driving current keeps flowing from the common power supply line xe2x80x9ccomxe2x80x9d into the luminescent element 40 and thus the luminescent element 40 keeps emitting light.
However, in the display device described above, unlike liquid crystal display devices, the opposite electrode xe2x80x9copxe2x80x9d opposing the pixel electrodes 41 is formed on the same transparent substrate 10 on which the pixel electrodes 41 are formed, such that the opposite electrode xe2x80x9copxe2x80x9d extends over the entire surface of the transparent substrate 10 or over the plurality of pixel regions 7, and thus there is only a second insulating film 52 between the opposite electrode xe2x80x9copxe2x80x9d and the data lines xe2x80x9csigxe2x80x9d. As a result, the data lines xe2x80x9csigxe2x80x9d have a large parasitic capacitance which causes the data lines xe2x80x9csigxe2x80x9d to have a large load. Similarly, a large parasitic capacitance is present between the opposite electrode and interconnection layers included in the data line driving circuit 3 or the scanning line driving circuit 4, because the opposite electrode xe2x80x9copxe2x80x9d extends over the data line driving circuit 3 and the scanning line driving circuit 4. As a result, the data line driving circuit 3 also has a problem of a large load caused by the large parasitic capacitance.
The inventor of the present invention has developed a technique of forming an organic semiconductor film in a desired area by emitting a liquid material from an ink-jet head. The inventor has also developed a technique of defining an area where an organic semiconductor film is to be formed by surrounding the area by a bank layer so that the organic semiconductor film can be formed precisely in the defined area by means of the ink-jet technique without producing a part protruding outward from the defined area Herein, the inventor presents a technique of solving the above-described problems using the above techniques.
That is, it is an object of the present invention to provide a display device including organic semiconductor films formed on a substrate, in particular areas defined by a bank layer thereby preventing data lines and driving circuits from having parasitic capacitance.
According to an aspect of the present invention, to achieve the above object, there is provided a display device comprising elements formed on a substrate, the elements including: a plurality of scanning lines; a plurality of data lines extending in a direction crossing the direction in which the scanning lines extend; a plurality of common power supply lines extending in a direction parallel to the data lines; and pixel regions formed in the shape of a matrix defined by the data lines and the scanning lines, each pixel region including: a first TFT having a gate electrode to which a scanning signal is supplied via one of the scanning lines; a holding capacitor for holding an image signal supplied from a corresponding data line via the first TFT; a second TFT having a gate electrode to which the image signal held by the holding capacitor is supplied; and a luminescent element including an organic semiconductor film formed between a pixel electrode provided in each pixel region and an opposite electrode extending across the data lines such that the opposite electrode faces the plurality of pixel electrodes, the luminescent element being adapted to emit light when the organic semiconductor film is driven by a driving current which flows between the pixel electrode and the opposite electrode when the pixel electrode is electrically connected to a corresponding common power supply line via the second thin film transistor, wherein light emitting areas of the organic semiconductor film are surrounded by a bank layer made up of an insulating film having a thickness greater than that of the organic semiconductor film, the bank layer being formed such that the data lines are, at least partly, covered with the bank layer.
In the present invention, because the opposite electrode is formed at least over the entire pixel regions or in the shape of stripes over a wide area, the opposite electrode faces the data lines. This can result in a large parasitic capacitance associated with each data line. However, in the present invention, the bank layer formed between the data lines and the opposite electrode prevents a large parasitic capacitance from occurring between the opposite electrode and the data lines. This results in a reduction in the load of the data line driving circuit. Therefore, it is possible to achieve a reduction in power consumption and an increase in the speed of displaying operation.
In the present invention, a first driving circuit for outputting the image signal over the data lines or a second driving circuit for outputting the scanning signal over the scanning lines may be formed, together with the plurality of pixel regions, on the substrate. If such a driving circuit is formed at a location facing the opposite electrode, the interconnection layer formed in the driving circuit also has a large parasitic capacitance. In the present invention, to avoid such a problem, the driving circuit is covered with the bank layer so as to prevent the driving circuit from having such a large parasitic capacitance with respect to the opposite electrode. This results in a reduction in the load of the driving circuit. As a result, a reduction in power consumption and an increase in the speed of displaying operation are achieved.
In the present invention, the organic semiconductor film may be a film formed, by means of an ink-jet technique, in areas surrounded by the bank layer, wherein the bank layer may be a water repellent film capable of preventing the organic semiconductor film from protruding outward from the above-described areas during the process of forming the organic semiconductor film by means of the ink-jet technique. In order to ensure that the organic semiconductor film is prevented from protruding outward, the bank layer may be formed to a thickness as large as about 1 xcexcm. In this case, to achieve a good partition wall, the organic semiconductor film is not necessarily required to be water repellent.
In the present invention, it is desirable that an area of each pixel electrode overlapping the corresponding first thin film transistor or second thin film transistor be also covered with the bank layer. In the present invention, in those areas of the pixel electrodes overlapping the first thin film transistors or the second thin film transistors, even if a driving current is passed through the organic semiconductor film to the opposite electrode and light is emitted from the organic semiconductor film, the light is blocked by the first TFT or the first TFT, and thus such light emission does not make a contribution to displaying of an image. The driving current flowing in such a particular area of the organic semiconductor film which makes no contribution to the display is herein referred to as an useless current. In the present invention, the bank layer is formed in those areas where an useless current otherwise would occur so that no useless current flows in the areas. As a result, it becomes possible to reduce the current flowing through the common power supply lines. Thus, it is possible to reduce the width of the common power supply lines thereby increasing the light emitting areas. This allows an improvement in the displaying performance such as brightness and contrast.
In the present invention, the bank layer is preferably formed of a black resist film so that the bank layer also serves as a black matrix which results in an improvement in the displaying performance. In the display device according to the present embodiment, the opposite electrode is formed at least over the entire pixel regions or into the shape of stripes over a wide area, and thus light reflected by the opposite electrode can cause degradation in the contrast. In the present invention, the above problem is avoided by forming the bank layer of the black resist serving as a black matrix, which also serves to prevent parasitic capacitance. That is, the bank layer blocks light reflected from the opposite electrode and thus the contrast is improved.
In the present invention, the driving currents for driving the respective luminescent elements are passed through the common power supply lines and thus the magnitude of the current passed through each common power supply line is greater than that flowing through each data line. Therefore, in the present invention, it is desirable that the resistance of each common power supply line per unit length be smaller than the resistance of each data line per unit length so that the common power supply line has a large current capacity. For example, when the common power supply lines and the data lines are made of the same material and they have the same thickness, the common power supply lines preferably have a larger width than the data lines.
In the present invention, it is desirable that pixel regions be disposed at both sides of each common power supply line such that they are supplied with the driving current via this common power supply line, and furthermore, the data lines extend over the pixel regions on the sides opposite to the common power supply line. That is, elements are disposed into a pattern periodically repeated in the direction along the scanning lines wherein the unit pattern consists of a data line, pixels connected to that data line, a common power supply line, pixels connected to that common power supply line, and a data line supplying an image signal to those pixels. In this arrangement, only one common power supply line is required for every two lines of pixels. Compared to the case where one common power supply line is formed for each line of pixels, the total area needed for the common power supply lines becomes small. As a result, it becomes possible to increase the light emitting area and thus the displaying performance such as brightness and contrast is improved.
In the construction described above, because two data lines are located close to each other, crosstalk can occur between these two data lines. In the present invention, to avoid the above problem, an interconnection layer is preferably formed between these two data lines. In this arrangement in which another interconnection layer different from the two data lines is disposed between the two data lines, the crosstalk can be prevented simply by maintaining the interconnection layer at a constant voltage at least for one horizontal scanning period.
In the present invention, in the case where the organic semiconductor film is formed by means of the inkjet technique, it is desirable that the center-to-center pitch of the organic semiconductor film areas be equal in any pixel regions arranged in the direction in which the scanning lines extend. In this case, it is simply required to emit an organic semiconductor material from an ink-jet head at equal intervals along the direction in which the scanning lines extend. This makes it possible to emit the organic semiconductor material exactly at specified points with a simple positioning control system.